Process for manufacturing tough steel



Feb. 10, 1970 susuMU GQDA ETAL 3,494,808

PRocEss Eon HANUFAGTUR'ING TOUGH STEEL Filed Jan. 14, 1964 vson ooo mosoo ooo-looonoo lzoo'laoo Heating temperaiure C v Acldinsoluble nlobium/fetal nobium Z by wt Q `1 N VEN Tons.

( WM Sfax/M da United States Patent O 3,494,808 PROCESS FORMANUFACTURING TOUGH STEEL Susumu Goda, Hisashi Gondo, Isao Kimura,Suehiro Hiyoshi, Akira Yonei, and Hiroki Masumoto, Kitakyushu, Fukuoka,Japan, assignors to Yawata Iron & Steel Co., Ltd., Tokyo, Japan, aJapanese corporation Filed Jan. 14, 1964, Ser. No. 337,562 Claimspriority, application Japan, Jau. 18, 1963, 38/ 2,228 Int. Cl. C21d7/14; C22c 39/30 U.S. Cl. 148-12 4 Claims This invention relates to aprocess for manufacturing tough steel, more particularly, a process formanufacturing tough steel above 15 mm. thick for use as structuralsteel.

Heretofore, a method is known of improving the strength of tough steelby adding niobium or vanadium thereto. However, the tough steel preparedby this known method loses rapidly the toughness to such an extent, Whenthe thickness of the steel is increased, that it becomes diflicult toobtain an impact value at C. according to the V-notch Charpy impacttesting method as of above 3.0 kgm./cm.2, which is generally requiredfor use in structural steels. Therefore, it is not on record that anytough steel above mm. thick has been practically used as structuralsteel, and the manufacture of tough steel of such thickness has beenregarded as almost impossible.

As conventional tough steel is prepared by heating and rollingtreatments at a temperature of above 1,250 C., bainites increase in itscrystal texture, when the thickness of the steel exceeds 15 mm., wherebythe toughness of the steel is lost rapidly :without effecting refinementof crystal grains.

To cite an example, a steel having the thickness of 25 mm. andcontaining one of niobium, tantalum and vanadium, which has been treatedat a temperature of above 1,250 C., shows an impact value of 1 to 1.5kgm./ cm?. Accordingly, the steel of this kind has been deemed to beunsuitable for wide use as a tough steel.

The present invention is based on the observation that certainrelationships exist between acid-soluble niobium and acid-insolubleniobium contained in steel. That is, acid-soluble niobium, which ismainly solid-dissolved in the steel, is the cause of improving thestrength of the steel due to hardening caused by the solid-solution, andacid-insoluble niobium, which is deposited mainly as carbide and partlyas nitride, is that which refines the crystal grains of the steel andthereby improves the toughness thereof. In analyses made by theinventors the fact has been discovered that both kinds of niobiums keepthe specific equilibrium values between them, which are mainly to bedetermined by the heating temperature and heating time and which undergosubstantially no change during the rolling process and the coolingprocess after the rolling, unless intentionally slow cooling is carriedout. This fact is entirely unknown in any known literature.

In addition, according to the literature it was said that niobiumcompounds contained in the steel would be completely decomposed andsolid-dissolved at a temperature of about 900 C., when they are in thecomposition range as specified by the present invention. However, by thepresent invention it Was found out that acid-insoluble niobium willincrease as the heating temperature will be higher and reach the maximumat a temperature of 900 ICC C. and then will begin to decreaseimmediately before 1,100 C.

Thus, the present invention is the first in obtaining the deposition ofniobium compounds at such a high temperature as above mentioned and inutilizing the effects thereof. The same phenomenon may be perceived alsoin case of tantalum and vanadium. The present invention has succeeded inachieving both the desired strength and toughness of the steel bycontrolling the solid-solution and deposition of these elements, andthereby the manufacture of a tough steel above 15 mm. thick, which hasbeen heretofore regarded as almost impossible, was made possible.

The object of the present invention is to provide a method ofmanufacturing a tough structural steel which possesses both propertiesof strength and toughness.

Another object of the present invention is to provide a method ofmanufacturing tough steel above 15 mm. thick which possesses bothproperties of strength and toughness.

Other objects of the present invention will be made clear by thefollowing description and in reference to` the attached drawing.

The single figure of drawing shows an equilibrium curve between theacid-insoluble niobium deposited in the steel and the total niobium atvarious heating temperatures.

The steels to be used in the present invention are those obtained byadding one or more selected from the group consisting of niobium,tantalum and vanadium in an amount of 0.02% to 0.20% in total to anordinary carbon steel containing less than 0.40% C, Si and Mn in anamount to be contained in making steel respectively, or to aferrite-pearlite steel similar to the above ordinary carbon steel. Incase higher strength is particularly required, nitrogen should becontained in au amount less than 0.02%.

The steel containing more than 0.4% C is not suitable for the presentinvention, because C content of more than 0.40% will reduce the impactvalue. The contents of Si and Mn should be limited to those incidentalto the manufacture of steel. If one or more of niobium, tantalum orvanadium is or are contained in an amount less than 0.02% in total, theeffect of adding these elements cannot be displayed. If more than 0.20%,the impact value will be lowered.

According to the method of the present invention, in which the steelspecified as above is heated and hotrolled at the specifiedtemperatures, a tough steel having sufficient low temperature strengthand toughness may be obtained, even if the thickness of the steelexceeds 15 mm. Even heretofore, a tough steel itself, to which niobium,tantalum and vanadium Were added, could be obtained by hot-rolling thesteel to the desired dimension and shape. However, in the conventionalmethod, the heating temperature was above 1,250" C. in general, and therolling temperature has been also determined only in View of theeasiness of plasticity.

On the other hand, the finish temperature of rolling was above 950 C. incase of rolling a relatively largesized steel and about 900 or 850 C.even in case of rolling small-sized one.

The present invention is characterized by controlling the amount ofsolid-solution of acid-soluble niobium and the amount of deposition ofacid-insoluble niobium.

'I'hat is to say, as shown in the aforesaid curve, acidinsoluble niobiumincreases gradually in accordance with the rise in the heatingtemperature and reaches the maX- imum at 900 C. and then again decreasesgradually at a temperature range of immediately before 1,100 C.

Therefore, it is possible to effectively control the amount ofdeposition by regulating the heating temperature.

As already mentioned, acid-insoluble niobium is deposited mainly in theform of carbide or partly of nitride. But, the amount of deposition isdesirable to be 30% t0 80% by weight, or more preferably 50% by weightof the total niobium.

` As seen from the curve in the attached drawing, there are two sectionsin the curve which show the range of the deposition of acid-insolubleniobium in an amount of 30% to 80% of the total niobium, correspondingto the range of the heating temperature of 550 to 800 C. and that of1,100 to 1,?.20v C., respectively. Correspondingly, the inventors havediscovered two methods to attain the object of the present invention,that is, to obtain a tough steel possessing both properties of strengthand toughness, irrespective of the thickness thereof, in whichacid-insoluble niobium is deposited in an amount of 30% to 80% byIweight of the total niobium. In general, the above mentioned twomethods are as follows:

I. The steel slab containing less than 0.40% C by weight, less than 1.5%Mn by weight and one or more selected from the group consisting ofniobium, tantalum and vanadium in an amount of 0.02% to 0.20% by weightin total is heated at a temperat-ure of 1,100 to 1,220 C. for at least15 minutes and immediately thereupon the thus heated slab is hot-rolled.

II. The steel slab specified as above mentioned is treated according tothe conventional method, in which it is heated at a temperature above1,250 C. and then the thus heated slab is hot-rolled, and then thehot-rolled slab is heated again at a temperature of 550 to 800 C. for atleast l5 minutes.

-By either of these two methods a tough steel of more than 15 mm. thick,which possesses both properties of strength and toughness, could beobtained.

The above steel slab may be prepared by the conventional method, inwhich a molten steel containing the elements as specied in the presentinvention is cast into an ingot and the ingot is made to a slab.

The time of heating the slab at a temperature of 1,100 to 1,220 C. or of550 to 800 C., that is, at least 15 minutes, is a time necessary foreffecting the deposition of acid-insoluble niobium or tantalum orvanadium in an amount of 30% to 80% by weight respectively.

The present invention will be explained more in detail by the example.Table I shows chemical compositions of specimens of steels prepared by abasic openhearth furnace, and Table II the relationships between thecompositions of steels and the impact values at 0 C. of said steelstreated as follows:

The steels were heated at a temperature of 1,200 C. and immediatelythereupon hot-rolled with a tinish temperature of 900 C.

In the specimens D, F and G nitrogen has been added. No nitrogen hasbeen added in the specimens which contained less than 0.006% N2.

TABLE IL MECHANICAL PROPERTIES (WHEN THE STEEL HAS BEEN HEATED AT 1,200C. AND ROLLED WITH A FINISH TEMPERATURE OF 900 C.) 25 MM. THICK As seenfrom the above table manganese improves the impact value withoutimpairing the strength. However, if the content of manganese is morethan 1.5 the steel becomes brittle and the weldability will be impaired.Therefore, it should be limited to below 1.5 by Weight. Nitrogenimproves strength of the steel, but will reduce the impact value when itincerases, and makes the steel brittle when the content thereof exceeds0.02%. Therefore, if the steel possesses already the desired strength,it is not necessary to especially add nitrogen. Carbon will reduce theimpact value, if the content thereof is more than 0.40%

On the other hand, even if the other elements are contained in thespecitied ranges as above mentioned, the addition of niobium, tantalumor vanadium of more than 0.20% -by weight in total will reduce largelythe impact values, while no effect of adding these elements will bedisplayed, if the addition of any of them is less than 0.02% by weightin total.

Table III shows the relationships between the heating and rollingconditions of specimen No. E and mechanical properties thereof.

Heating temperature, C.

Finish temperature of rolling, C. 1, 100 1, 150 1, 200 1, 220 1, 250

(2) 52 (2) 53 (2) 54 (2) 59 (l) 51 52 (6) 54 (4) 55 (4) (1) 52 (15) 53(l0) 55 (8) 56 (4) (l) 52 (14) (10) 55 (8) (3) 61 (1) 53 (15) 54 (11) 56(10) 57 (4) 60 (1) 54 (15) 55 (13) 57 (9) 56 (4) 61 (2) Numbers inparentheses designate the impact value at 0 C. (kg. m./cm.2).

Numbers outside parentheses show the tensile strength (kg/mm2).

As seen from Table III, if the heating temperature eX- ceeds 1,220 C.,the tensile strength Will be elevated, but

TABLE I.-CHEMICAL COMPOSITIONS (PERCENT BY WEIGHT) Tantalum VanadiumNitrogen Carbon Silicon Manganese Nobium AFTER ROLLING AND MECHANICALPROPERTIES (EXAMPLE OF E) 25 MM. THICK Reheatng temperature beforerolling, 1,250 C. Final temperature of rolling, 900 C.

Tensile strength Impact value (kg/mm 2) (kg. rn./cm.2)

Reheating temperature 0.):

As welded 61 1 00 59 1 58 2 57 4 54 10 53 14 51 15 As seen from Table1V, in the slab, which has been subjected to a heating treatment at atemperature of 1,250 C. as according to the conventional method, theratio of amount of deposition to amount of solid-solution of niobium,tantalum or vanadium necessary for obtaining both strength and toughnessas aimed at by the present invention could not be obtained.Consequently, the impact value thereof was very low.

Therefore, in order to Obtain 30% to 80% of the deposition ofacid-insoluble niobium, tantalum or vanadium, it was necessary to reheatthe slab heated and hot-rolled according to the conventional method at atemperature of 550 to 800 C. as specified by the present invention.

By thus reheating the slab both strength and toughness as desired couldbe obtained. But if the reheating temperature is not in the abovementioned range, the object of the present invention cannot be attained.

What we claim is:

1. A method for producing tough steel having a thickness exceeding 15mm. comprising heating steel consisting of less than 0.4% by weight C,less than 1.5% 4by weight Mn and from 0.02 to 0.20% by weight Ta, therest being essentially iron, at a temperature of 1,100 to 1,220 C., forat least 15 minutes, and thereupon subjecting the heated steel tohot-rolling with a finishing temperature of less than 1,000 C.

2. A method for producing tough steel having a thickness exceeding 15mm. comprising making molten steel consisting of less than 0.40% byweight C, less than 1.5% by weight Mn and 0.02 to 0.20% by Weight Ta,the rest being essentially iron, casting the molten steel into form,making a slab from the ingot, then heating said slab at a temperature of1,100 to 1,220 C. for at least 15 minutes, and thereupon subjecting theheated slab to hot rolling with a finishing temperature of below 1,000C.

3. A method for producing tough steel having a thickness exceeding 15mm. comprising heating steel consisting of less than 0.4% by weight C,less than 1.5% by weight Mn and from 0.02 to 0.20% by weight V, the restbeing essentially iron, at a temperature of 1,100 to 1,220 C., for atleast 15 minutes, and thereupon subjecting the heated steel tohot-rolling with a finishing temperature of less than 1,000 C.

4. A method for producing tough steel having a thickness exceeding 15mm. comprising making molten steel consisting of less than 0.40% byweight C, less than 1.5 by weight Mn and 0.02 to 0.20% by Weight V, therest being essentially iron, casting the molten steel into form, makinga slab from the ingot, then heating said slab at a temperature of 1,100to 1,220 C. for at least 15 minutes and thereupon subjecting the heatedslab to hot rolling with a finishing'l temperature of below 1,000 C.

References Cited The Making, Shaping and Treating of Steel, seventhedition, 1957, pp. 386-387.

L. DEWAYNE RUTLEDGE, Primary Examiner W. W. STALLARD, Assistant Examiner

1. A METHOD FOR PRODUCING TOUGH STEEL HAVING A THICKNESS EXCEEDING 15MM. COMPRISING HEATING STEEL CONSISTING OF LESS THAN 0.4% BY WEIGHT C,LESS THAN 1.5% BY WEIGHT MN AND FROM 0.02 TO 0.20% BY WEIGHT TA, THEREST BEING ESSENTIALLY IRON, AT A TEMPERATURE OF 1,100 TO 1,200*C., FORAT LEAST 15 MINUTES, AND THEREUPON SUBJECTING THE HEATED STEEL TOHOT-ROLLING WITH A FINISHING TEMPERATURE OF LESS THAN 1,000*C.